Browsing by Author "Snel, Zander"
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- ItemOptimised structural reliability as a basis for prestressed concrete design(Stellenbosch : Stellenbosch University, 2022-10) Snel, Zander; De Koker, Nico; Viljoen, Celeste; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Prestressed concrete design is performed via the limit states design approach. The design of a prestress configuration for flexure is generally governed by the serviceability limit state of cracking. The crack control criterion is enforced in design by means of four stress inequality equations. These four inequalities delimit a domain of feasible prestress configurations which is illustrated graphically as the well-known Magnel diagram. The goal of this study is to assess the performance of this design procedure in the context of cost optimised structural reliability. Magnel diagrams are used extensively in this study. The diagrams depict the domain of feasible design configurations, over which the predicted reliability performance is mapped. A probabilistic analysis model is developed to evaluate the reliability and life cycle economy of a reference structure. A single span concrete girder bridge is selected as reference structure with composite prestressed concrete sections. The reference structure is designed for traffic loading for 15, 20 and 25m span lengths. This study focuses on the design guidelines as provided in the Eurocodes. Evaluation of the reliability performance of the prestressed concrete serviceability limit state design methodology finds that the partial factors and resistance factors provided in the Eurocodes results in sub-optimal reliability performance. The achieved reliability index varies over the Magnel diagram with reliability indices increasing away from the Magnel diagram boundaries. Taking into account the economic considerations associated with the life cycle costs of a prestressed element shows that the cost optimal design location within the Magnel diagram is at the maximum practical eccentricity, located at a distance from both the top and bottom Magnel diagram boundaries. Further, the study demonstrates that for a given beam profile and aspect ratio, an optimal beam profile size is attainable. The partial factors and resistance factors for prestressed concrete serviceability limit state design are not reliability-based, which results in the sub-optimal reliability performance. The reliability performance of these factors is contrasted against the reliability performance of a set of partial factors for serviceability limit state design of prestressed concrete elements calibrated using the design value method described by the Eurocodes in EN 1990:2002. The developed set of partial factors significantly improves the reliability performance along the boundaries of the Magnel diagram. However, limitations of the design value method for the serviceability limit state problem of prestressed concrete are highlighted. These shortcomings are treated by means of guidelines regarding the selection of a prestress force and configuration.